1. Field of the Invention
The present invention relates to a method for preparing a semiconductor member, and more particularly to a method for preparing a semiconductor member suitable for the dielectric isolation, or an electronic device or integrated circuit created on a monocrystalline semiconductor layer on an insulating material.
2. Related Background Art
Formation of a monocrystalline Si semiconductor layer on an insulating material has been widely known as the silicon on insulator (SOI) technology, and since a large number of advantages which cannot be reached by bulk Si substrates for preparation of conventional Si integrated circuits are possessed by the device utilizing the SOI structure, so many researches have been done. More specifically, by utilizing the SOI structure, the following advantages can be obtained:
1 Dielectric isolation can be easily done to enable high degree of integration; PA1 2 Radiation hardness is excellent; PA1 3 Stray capacity is reduced to attain high speed; PA1 4 Well formation step can be omitted; PA1 5 Latch-up can be prevented; PA1 6 Fully depleted field effect transistor can be made by thin film formation. PA1 a monocrystalline Si layer is formed directly to lateral epitaxial growth by CVD; PA1 amorphous Si is deposited and subjected to solid phase lateral epitaxial growth by heat treatment; PA1 an amorphous or polycrystalline Si layer is irradiated convergently with an energy beam such as electron beam, laser beam, etc., and a monocrystalline layer is grown on SiO.sub.2 by melting and recrystallization; and PA1 a melting region is scanned in a zone fashion by a rod-shaped heater (Zone melting recrystallization). PA1 m1: Total weight before anodization PA1 m2: Total weight after anodization PA1 m3: Total weight after removal of porous Si PA1 .rho.: Density of monocrystalline Si PA1 A: Area of porous region PA1 t: Thickness of porous Si PA1 1. The method of etching porous Si with an aqueous NaOH solution (G. Bonchil, R. Herino, K. Barla, and J. C. Pfister, J. Electrochem. Soc., vol. 130, No. 7, 1611 (1983)); PA1 2. The method of etching porous Si with an etching solution which is capable of etching monocrystalline Si.
In order to realize the many advantages in device characteristics as mentioned above, studies have been made about the method for forming the SOI structure for these some 10 years. The contents are summarized in, for example, the literature as mentioned below:
Special Issue: "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Cullen, Journal of Crystal Growth, volume 63, No. 3, pp. 429-590 (1983).
Also, it has been known for a long time to form the SOS (silicon on sapphire) structure by heteroepitaxy of Si on a monocrystalline sapphire substrate by CVD (chemical vapor deposition) method. This was successful to some extent as the most mature SOI technique, but for such reasons as a large amount of crystal defects because of lattice mismatching at the interface between the Si layer and the underlaid sapphire substrate, introduction of aluminum from the sapphire substrate into the Si layer, and above all the high price of the substrate and delay in enlargement of the substrate wafer size, it is obstructed from being widely applied.
Relatively in recent years, attempts to realize the SOI structure without use of a sapphire substrate have been done. Such attempts may be broadly classified into the two technologies shown below:
1. After surface oxidation of an Si monocrystalline substrate, a window is formed to have the Si substrate partially exposed, and epitaxial growth is proceeded in the lateral direction with that exposed portion as the seed to form an Si monocrystalline layer on SiO.sub.2. (In this case, deposition of Si layer on SiO.sub.2 is accompanied).
2. By use of an Si monocrystalline substrate itself as an active layer, SiO.sub.2 is formed therebeneath. (This method is accompanied with no deposition of Si layer.)
As the means for realizing the above 1, there have been known the methods in which:
These methods have both advantages and disadvantages, they still have many problems with respect to controllability, productivity, uniformity and quality, and none of them have been industrially applied yet up to date.
For example, the CVD method requires the sacrificial oxidation in flat thin film formation, while the crystallinity is poor in the solid phase growth method.
On the other hand, in the beam annealing method, problems are involved in controllability such as treatment time by converged beam scanning, the manner of overlapping of beams, focus adjustment, etc.
Among these, the Zone Melting Recrystallization method is the most mature, and a relatively larger scale integrated circuit has been trially made, but still a large number of crystal defects such as subboundary remain, and no device driven by minority carriers has been prepared.
Concerning the method using no Si substate as the seed for epitaxial growth which is the above method 2, for example, the following three methods may be included.
1. An oxide film is formed on an Si monocrystalline substrate with V-grooves as anisotropically etched on the surface, a polycrystalline Si layer is deposited on the oxide film thick to the extent as the Si substrate, and thereafter by polishing from the back surface of the Si substrate, Si monocrystalline regions dielectrically separated by surrounding with the V-grooves on the thick polycrystalline Si layer are formed.
In this method, although crystallinity is good, there are problems with respect to controllability and productivity as it requires a process of depositing the polycrystalline Si thick as some hundreds .mu.m, and a process in which the monocrystalline Si substrate is polished from the back surface to leave only the Si active layer as separated.
2. This is the method called SIMOX (Separation by Ion Implanted Oxygen) in which an SiO.sub.2 layer is formed by ion implantation of oxygen into an Si monocrystalline substrate, which is one of the most mature methods at present because of good matching with the Si process.
However, for formation of the SiO.sub.2 layer, 10.sup.18 ions/cm.sup.2 or more of oxygen ions are required to be implanted, and the implantation time is very long to be not high in productivity, and also the wafer cost is high. Further, many crystal defects remain, and from an industrial point of view, no sufficient level of quality capable of preparing a device driven by minority carriers have been attained.
3. This is the method to form an SOI structure by dielectric isolation according to oxidation of porous Si. This is a method in which an N-type Si layer is formed on the surface of a P-type Si monocrystalline substrate in shape of islands by way of proton ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) , or by epitaxial growth and patterning; only the P-type Si substrate is made porous by anodization in the HF solution so as to surround the Si islands from the surface; and then the N-type Si islands are dielectrically isolated by accelerated oxidation.
In this method, the separated Si region is determined before the device steps, whereby there is the problem that the degree of freedom in device design may be limited in some cases.
Generally, on a light-transparent substrate represented by a glass, the deposited thin film Si layer is only formed as an amorphous layer or, at best, a polycrystalline layer because of reflecting the disorder of the crystal structure thereof, and it is therefore difficult to produce a high quality device. This is because the substrate has an amorphous crystal structure, and thus a monocrystalline layer of high quality cannot be easily obtained by simply depositing the Si layer.
By the way, the light-transparent substrate which is one of the insulating substrates is important for constituting a contact sensor serving as a light-receiving device and a projection-type liquid crystal image display, and a high-quality driving device is required for further increasing the density, resolution and definition of the pixels (picture elements) of such a sensor or display. It is consequently necessary to produce a device to be provided on a light-transparent substrate by using a monocrystalline layer having excellent crystallinity.
It is therefore difficult to produce a driving device having properties sufficient for the present demands or future demands because the crystal structure of an amorphous Si or polycrystalline Si has many defects.
Therefore, there is the problem that any of the abovementioned methods is difficult to provide an SOI layer having excellent crystallinity equal to that of the Si wafer on a light-transparent glass substrate which is one of the insulating substrates.
The removal of porous Si layer by chemical etching which is a requisite process for the method of the present invention will be described below.
In general, EQU P=(2.33-A)/2.33 (1)
is called the porosity. This value can be changed by anodization, and expressed as follows. EQU P=(m1-m2)/(m1-m3) (2)
or EQU P=(m1-m2)/.rho. A.sub.t ( 3)
However, the area of porous region cannot be accurately calculated in some cases. In such a case, although the expression (2) is effective, the porous Si must be etched for measuring the value of m3.
In addition, during epitaxial growth on the porous Si, the porous Si is capable of relieving distortion produced during heteroepitaxial growth and suppressing the occurrence of defects owing to its structural property. However, in this case, it is clear that the porosity of porous Si is a very important parameter. Therefore, the above-mentioned measurement of the porosity is necessary and indispensable in this case.
Known methods of etching porous Si are the following methods (1) and (2):
In the above method 2, an etching solution of fluoronitric type is generally used, and etching of Si proceeds as follows: EQU Si+20.fwdarw.SiO.sub.2 ( 4) EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +H.sub.2 O (5)
As shown, Si is oxidized by nitric acid to SiO.sub.2, and the SiO.sub.2 produced is etched by hydrofluoric acid.
Examples of etching solutions of crystalline Si include the above fluoronitric acid-type etching solution as well as ethylenediamine-type, KOH-type, and hydrazine-type etching solution and the like.
From these respects, it is necessary in selective etching of porous Si to select an etching solution which is capable of etching porous Si, other than the above Si etching solutions. Conventionally, the porous Si is generally selectively etched only by using an aqueous NaOH solution as an etching solution.
As above mentioned, both porous Si and monocrystalline Si are etched with the fluoronitric acid-type etching solution.
On the other hand, in the conventional method of selectively etching porous Si with an aqueous NaOH solution, Na ions are inevitably adsorbed on the etched surface.
Since this Na ions cause impurity contamination, are movable and have adverse effects such as the formation of interfacial states, the ions must not be introduced into the semiconductor process.